Activated metal catalysts are also known in the fields of chemistry and chemical engineering as Raney-type, sponge and/or skeletal catalysts. They are used, largely in powder form, for a large number of hydrogenation, dehydrogenation, isomerization, reductive amination, reductive alkylation and hydration reactions of organic compounds. These powdered catalysts are prepared from an alloy of one or more catalytically-active metals, also referred to herein as the catalyst metals, with a further alloying component which is soluble in alkalis. Mainly nickel, cobalt, copper, iron or combinations thereof are used as catalyst metals. Aluminum is generally used as the alloying component which is soluble in alkalis, but other components may also be used, in particular zinc and silicon or mixtures of these either with or without aluminum.
These so-called Raney alloys are generally prepared by the ingot casting process. In that process a mixture of the catalyst metal and, for example, aluminum is first melted and casted into ingots.
Typical alloy batches on a production scale amount to about ten to a couple hundred kg per ingot. According to DE 21 59 736 cooling times of up to two hours were obtained for this method. This corresponds to an average rate of cooling of about 0.2 K/s. In contrast to this, rates of 102 to 106 K/s and higher are achieved in processes where rapid cooling is applied (for example an atomizing process). The rate of cooling is affected in particular by the particle size and the cooling medium (see Materials Science and Technology edited by R. W. Chan, P. Haasen, E. J. Kramer, Vol. 15, Processing of Metals and Alloys, 1991, VCH-Verlag Weinheim, pages 57 to 110). A process of this type is used in EP 0 437 788 B 1 in order to prepare a Raney alloy powder. In that process the molten alloy at a temperature of 5 to 500° C. above its melting point is atomized and cooled using water and/or a gas.
To prepare a powder catalyst, the Raney alloy which can be made by a known process (i.e. according to EP 0 437 788 B1) is first finely milled, if it has not been produced in the desired powder form during preparation. Then the aluminum is partly (and if need be, totally) removed by extraction with alkalis such as, for example, caustic soda solution (other bases such as KOH are also suitable) to activate the alloy powder. These types of catalysts can be activated with most bases and acids to give varying results. Following extraction of the aluminum, the remaining catalytic power has a high specific surface area (BET), between 5 and 150 m2/g, and is rich in active hydrogen. The activated catalyst powder is pyrophoric and stored under water or organic solvents or is embedded in organic compounds (e.g., distearylamine) which are solid at room temperature.
U.S. Pat. No. 6,423,872 describes the use of Ni catalysts that contain less than 5.5 wt % Al for the hydrogenation of nitrated aromatics. It describes the use of both commercially available standard activated Ni catalysts and supported Ni catalysts for the hydrogenation of nitrated aromatics, where problematic nickel aluminates are formed during this hydrogenation if their Al content is 5.5 wt % Al or higher.
These nickel aluminates can be in the form of takovite and/or takovite-like compounds and all of these nickel aluminates need to be removed from the desired amine before it is processed further. These nickel aluminates tend to form solids in the reactor and in the peripheral equipment (e.g., piping, settling tanks, filtration equipment, pumps and other equipment used in this process) that can deposit on their walls to decrease their heat transfer efficiency and to create blockages in the system.
Hence the formation of these nickel aluminates creates both safety hazards and a drop in productivity. The buildup of these nickel aluminates make it difficult to continue with the reaction and in such cases, one needs to shutdown the plant and clean out these deposits from the reactor and the peripheral equipment.
U.S. Pat. No. 6,423,872 also mentions the use of very specific alloy dopants limited to a definite list of elements that remain in the activated Ni catalyst after activation with caustic and the use of these resulting catalysts for the continuous hydrogenation of nitrated aromatics.
The conventional alloy doping elements from the groups IVA, VA, VIA and VIII of the periodic table of elements were specifically claimed in this patent. Additional Alloy doping elements such as titanium iron and chromium were also claimed.
U.S. Pat. No. 6,423,872 describes the use of a Ni catalyst having less than 5.5 wt % Al for the continuous hydrogenation of nitrated aromatics due to its lower formation of undesirable nickel aluminates during this hydrogenation. In principle, the less Al you have in the catalyst, the lower the amount of the nickel aluminates you will form. However these catalysts still form nickel aluminates and this technology does have its limits since the Al that is present in them is still considerably leachable under the conditions used for the hydrogenation of nitro-compounds such as nitrated aromatics.
U.S. Pat. No. 6,423,872 keeps the Al level lower than 5.5 wt % by changing the Al content of the alloy and/or increasing the intensity of the activation process. Increasing the Al content in the alloy will increase the amounts of Al-rich and more readily leachable phases such as NiAl3 and the Al-eutectic phases. Another way to increase the amounts of these phases would be to perform the appropriate heat treatment to the alloy either after or during its production. Increasing the amounts of these readily leachable phases can also decrease the mechanical stability of these catalysts, thereby leading to a lower lifetime for the catalysts.
Hence lowering the Al content of the catalyst simply by increasing the amount of leachable phases in the precursor alloy does have its limitations.
Another method that U.S. Pat. No. 6,423,872 describes to decrease the Al content in the catalyst was to increase the intensity of the activation process by increasing the leaching temperature, pressure and other parameters that accelerate this process. However, not only does this increase the cost of the catalyst, but it also produces a sodium aluminate side product that is not salable and would need to be disposed of. Moreover if one is not careful during leaching, the newly formed sodium aluminate under these harsher conditions may deposit back on to the catalyst and block its catalytically active surface leading to lower activity and shorter catalyst life.
While the methods of U.S. Pat. No. 6,423,872 do decrease the level of leachable Al to some degree, they do not entirely solve the problems involved with the hydrogen of nitro-compounds, since most alloy activations used in catalyst production occur under different conditions than those of the continuous hydrogenation of nitro-compounds such as nitrated aromatic compounds. Thus the commercially applicable methods of U.S. Pat. No. 6,423,872 produce a catalyst that still has a considerable amount of Al in the catalyst that can be leached out during the hydrogenation of nitrated aromatic compounds.